How to convert vertex group to sculpting mask?
It's possible to achieve that with Dyntopo enabled and Mask Tools addon installed (see Installing an addon) and activated. 

With addon, in Sculpt mode press Vertex Group To Mask button on the Toolshelf > Sculpt rollout.



Dyntopo allows that because the geometry needed for vertex group will be dynamically created while using Sculpt brush.
The Multires modifier isn't available for that unfortunately.